The present invention relates in general to apparatus for monitoring television receivers to obtain information for the rating of television programs.
Heretofore, monitoring apparatus had detected the local oscillator frequency generated within the monitored television receiver. While the detection of the local oscillator frequency of the television receiver offered a positive identification of the channel to which a monitored television receiver was tuned, the measurement of the local oscillator frequency generally was not reliable or accurate. Examples of monitoring systems for television receivers employing the detection of local oscillators in the television receiver being monitored are disclosed in the following U.S. Pat. Nos. 2,965,433; 3,456,192; 3,506,916; 3,299,355.
It has been heretofore known to have a monitoring apparatus for monitoring a television receiver in which a comparison was made between the vertical and/or horizontal synchronization signals from the monitored television receiver and the corresponding signal demodulated from a known television channel. The latter signal was obtained from a television tuner in the monitoring apparatus. It has been found that several transmitters broadcasting the same television program over different channels would transmit essentially the same synchronization pulses. As a consequence thereof, it was difficult to distinguish between the two channels in recording the monitoring information. If an attempt were made to detect very small phase differences in the synchronization pulses, then false indications resulted.
An example of a monitoring apparatus for television receivers employing vertical and/or horizontal synchronization signals is disclosed in the following U.S. Pat. No. 3,372,233.
Another system previously known employed the measurement of the input impedance of the tuner of the television receiver. The input impedance of the tuner of the television receiver offered a positive identification of the channel selected. In this system, an R.F. signal was injected into the input of the tuner of the television receiver. The injected R.F. signal was, in effect, superimposed on the television signal detected by the antenna. As a consequence thereof, the quality of the picture of the television picture tube had been affected.
A commonly known monitoring technique, which is disclosed in the patent to Krahulec, U.S. Pat. No. 2,838,359, was the detection of the position of the tuning shaft of the tuner of the television receiver. This arrangement was derived from the fact that the frequency of the local oscillator was an indication of the channel selected by the monitored television receiver.
Various modulating techniques for monitoring a television receiver have been disclosed in the following U.S. Pat. Nos. 2,892,885; 3,312,900; 2,935,557; 3,397,402; 2,864,941; 3,703,684.
Disclosed in the patent to Watanabe, U.S. Pat. No. 3,803,349, issued on Apr. 9, 1974, is a monitor for television receivers in which a comparison is made between a sound intermediate frequency signal from a monitored television receiver and sound intermediate frequency signals successively received by a monitoring apparatus having a tuner that is successively tuned to the various channels. The tuner of the monitoring apparatus scans the channels of television programs until it detects the same channel to which the monitored television receiver is tuned. It appears, however, that the sound intermediate frequency signal is not readily extractable from a television receiver and that it is difficult to physically locate the sound IF frequency for extracting the same.
In the patent to Clark, Jr., U.S. Pat. No. 2,483,573, a comparison is made between the local oscillator of a radio receiver and an oscillator sweeping through the radio frequency range of the receiver for detecting the station to which the receiver is tuned. The position of a tuning capacitor that varies the frequency of the sweeping oscillator identifies the station to which the radio receiver is tuned.
The patent to Joseph Wu, U.S. Pat. No. 3,736,510, issued on May 29, 1973, for a Frequency And Modulation Monitor, disclosed apparatus for detecting intercarrier spacing errors in which a period comparator circuit compares the period between the incoming signal from the reference divider and the incoming signal from the variable program divider to detect a difference in periods for determining the intercarrier spacing error.
Other patents of interest are as follows: U.S. Pat. Nos. 2,958,766; 3,058,065; 3,143,705; 3,760,275; 3,849,729; 3,034,707; 2,917,622; 3,483,327; 3,318,517; 3,126,513.